1. Field of the Invention
The present invention relates to a fixing device in an image forming apparatus employing an electro-photographic process such as a laser printer, an electro-photographic copying machine and the like, and particularly, to a fixing device comprising a heating roller according to a system in which a plurality of heater lamps different in light-distribution characteristics are disposed internally and the surface is divided in region in the lengthwise direction to effect heating, and a control method therefor.
2. Related Art Statement
In an Image forming apparatus 1 employing an electro-photographic system such as a laser printer, an electro-photographic copying machine and the like, as shown in FIG. 1, first, the surface of a photosensitive drum 2 is charged by a charging device 3 to a fixed surface potential, then the surface of the photosensitive drum 2 is exposed by an exposing device 4 such as a semiconductor laser, and the surface potential is decayed to form an electrostatic latent image. And, a bias voltage is applied to the surface of a developing roller 6 of a developing device 5, toner charged within the developing device 5 is adhered to an electrostatic latent image forming section on the surface of the photosensitive drum 2, developed and actualized to form a toner image on the surface of the photosensitive drum 2.
On the other hand, paper P is taken out of a paper cassette 7 by a paper-feed roller 8 and carried by carrying rollers 9, 9, the paper P is charged by a transfer device 10, and a toner image formed on the surface of the photosensitive drum 2 is transferred to the surface of the paper P. Next, the paper P is stripped from the surface of the photosensitive drum 2 by a stripping device 11, the paper P is held by a heating roller 13 and a pressing roller 14 of a fixing device 12, heated and pressed to thereby fixed the toner image on the surface of the paper P, and thereafter the paper P is discharged outside the device by a paper discharging roller 15.
As the aforementioned heating roller 13 of the fixing device 1, there is employed one in which a plurality of heater lamps 17, 18 are disposed internally of an annular tube 16 as shown in FIG. 2. However, recently, the heating roller 13 having a wall-thickness of the annular tube 16 thinned is being used in order to shorten the warming up time.
However, there was a problem that if the wall-thickness of the annular tube 16 is made thin, a change in temperature of the surface of the annular tube 16 becomes great, because of which immediately after termination of the warming up operation, unevenness in temperature occurs in the lengthwise direction of the heating roller 13, whereby poor fixing occurs in an image to be formed on the surface of paper P.
In continuous copying, since paper P being carried takes heat from the surface of the annular tube 16, a difference in temperature occurs in the lengthwise direction of the heating roller 13 depending upon what width of paper P is carried. Particularly, where paper P of small width is carried such as where paper P of A4 size is carried in the longitudinal direction, a temperature abnormally elevates at both ends of the heating roller 13 through which paper P does not pass, whereby parts constituting the heating roller 13 and parts disposed in the periphery thereof become broken; and in copying plural sheets, there was a problem also that a poor image occurs such that a high temperature offset image is formed on the surface of paper P.
For overcoming the problems as noted above, it is contemplated an employment of a heating roller 13 in accordance with a system in which two heater lamps 17, 18 different in light-distribution characteristics are disposed internally of an annular tube 16, and the surface of the heating roller 16 is divided in region in the lengthwise direction to effect heating.
However, even if such a heating roller 13 is employed, when the temperature of the surface of the heating roller 13 is set high in order to overcome the unevenness of temperature in the lengthwise direction of the heating roller 13 immediately after termination of the warming up operation, poor fixing of an image is improved, but there was a problem that the warming up time is unavoidably long.
Further, two heater lamps cannot be lighted simultaneously depending upon the operating conditions due to a problem of flicker or the like, a need occurs to execute a control that the amps are lighted alternately. However, in such a case as described, it is necessary to set the temperature of the surface of the heating roller 13 high immediately after termination of warming up operation in order to prevent poor fixing of an image, thus posing a problem that the warming up time is unavoidably long.